There’s a collection in Progress in Earth and Planetary Science lately (vol. 12):
Vernazza, P. Jorda, L. Tardivel, S. et al. Surface science on Phobos with the navigation cameras of the MMX IDEFIX rover 46 s40645-025-00708-4
Yokota, S. Matsuoka, A. Murata, N. et al. Pre-flight performance of the ion energy mass spectrum analyzer for the Mars Moons eXploration (MMX) mission 51 s40645-025-00718-2
Knollenberg, J. Grott, M. Hamm, M. et al. The miniRAD instrument for the MMX IDEFIX rover 53 s40645-025-00717-3
Murdoch, N. Lalucaa, V. Sunday, C. et al. The WheelCams on the IDEFIX rover 54 s40645-025-00725-3
These articles concern the MMX mission to Phobos and Deimos by JAXA, and specifically, three of them concern the Idefix rover, a Franco–German co-contribution to the mission.
Starting from the simplest, we have the main cameras aboard the Idefix rover. In addition to just driving, the images returned will have science value. This paper elaborates that value.
The main MMX orbiter/lander, even before dropping Idefix onto Phobos, will have already taken data on Phobos (and Deimos). One of the instruments is a plasma detector, with magnetometer. Phobos and Deimos are bombarded constantly, by cosmic rays, the more energetic fractions of the solar wind, and by micrometeorites. This results in a faint emission of Phobos/Deimos material, which eventually results in a Phobos torus and Deimos torus around Mars. By ‘sniffing’ the material, we can deduce things about the two satellites, and by the way constrain the atmosphere of Mars (Phobos is orbiting at a low altitude). One of the more exciting prospects: Phobos may have a water content, below its dusty surface, and this water may escape (feebly) into the Phobos torus. If MMX truly sniffs nontrivial Phobos water, the satellite would instantly become valuable as a base for refueling/restocking Mars missions.
Next comes Knollenberg et al., describing the rover’s infrared instrument miniRAD. Besides the driving cameras described above, the infrared, multi-band detector will sense the temperature of Phobos ahead of Idefix. From temperature, we can deduce further properties of the regolith. And some minerals may reveal themselves, by their infrared signatures.
The rover has more cameras, pointed down at the wheel/surface interface. By imaging the wheel tracks, we can say something about the properties of the regolith. Certainly, grain sizes will be noted by all cameras. But the wheel tracks tell us of the soil mechanics. The depth the wheels sink in the surface tells us of the particle cohesion, and their response in the miniscule Phobos gravity.
All in all, having ‘ground truth’ from the rover aids the overall mission. As with all sample return programs (and the atmospheric probe of Galileo), there’s the possibility that the sample gets taken from a non-representative spot. We want to know how the sampled spot looks compared to other spots on Phobos, and if it’s not ‘average’ (an average we don’t quite know yet), how does it differ? And of course, there’s always the possibility of a surprise- the ‘exploration’ in Mars Moons eXploration.
…and after all, what’s the point of MMX? The origin of Phobos and Deimos is debatable. Two possible histories are that they are captured asteroids, or that an impact threw Mars material into orbit. That material eventually accreted into an upper and lower satellite. Either way, Phobos and Deimos, as bodies a few kilometers in size, make good stand-ins for (other?) asteroids.